The invention relates to a liquid crystal color display screen provided with an electro-optical medium, two parallel transparent substrates by which the electro-optical medium is flanked, means to influence the transmission state of the electro-optical medium, a phosphor layer comprising at least two phosphors, which is situated on the first substrate, and a radiation source for generating radiation having a maximum emission at a wavelength xcex1 less than 360 nm, which radiation source is situated at the side of the second substrate.
The principle of a liquid crystal display screen is based on the fact that, by applying an electric field, the molecular orientation of liquid organic substances, behaving optically like birefringent crystals, can be controlled such that the direction of polarization of extraneous, incident, linearly polarized light is influenced.
A conventional liquid crystal display screen is generally composed of two glass plates, which are each coated at the inside with a transparent electrode of indium tin oxide (ITO). In the case of TN-liquid crystal display screens (TN=twisted nematic), a 90xc2x0 rotated edge orientation between the two plates is imposed on the liquid crystal molecules by orientation layers. As a result, a 90xc2x0 helix arises in the liquid crystal molecules. Crossed polarizers on the outer surfaces of the glass plates and a two-dimensional backlighting complete the display screen. As long as no electric voltage is applied to the two ITO electrodes, the light originating from the backlighting, which is linearly polarized by the first polarizer, can follow the rotation through 90 degrees of the liquid crystal molecules and, subsequently, pass through the second polarizer; the display screen appears transparent. If a sufficiently high voltage is applied, the electric anisotropy of the liquid crystal molecules causes the helix to be removed and the direction of polarization remains uninfluenced. The polarized light cannot pass through the second polarizer, and the cell appears dark.
A complete picture is composed of a plurality of individual elements, which are each driven as light valves by means of a matrix. As regards the drive, a distinction is made between passive and active matrices. At present, the majority of the liquid crystal display screens produced worldwide are driven by a passive matrix, rendering necessary the use of transparent strip electrodes on both glass plates. In the case of liquid crystal display screens driven by an active matrix, each pixel is associated with a switch of its own, which may consist of a thin-film transistor (TFT) or a thin-film diode (TFD). Liquid crystal display screens comprising an active matrix exhibit, all in all, a better contrast and a higher color saturation in combination with a shorter rise time. In the case of colored liquid crystal display screens, each pixel is composed of three, individually driven elements for the colors red, blue and green. In conventional liquid crystal display screens, mosaic color filters, which are pressed onto the front glass plate, are responsible for the color rendition.
A drawback of the conventional liquid crystal color display screens comprising color filter layers resides in that the display screen can only be looked at from specific viewing angles, and the color saturation, luminous intensity and brightness are clearly inferior as compared to CRT display screens.
Liquid crystal color display screens comprising a phosphor layer have a higher luminous intensity and a larger viewing angle.
For example, U.S. Pat. No. 4,822,144 discloses a liquid crystal color display screen which is operated in the transmission mode and is based on a combination of liquid crystal switching elements and a phosphor layer, said phosphor layer being excited by a UV light source, and the brightness of the display screen being increased by an interference filter between the light source and the phosphor layer. The phosphor layer and the UV source may be situated at two remote sides of the liquid crystal switching elements. The UV source may be a mercury high-pressure lamp, which emits light with a maximum emission in the range between 360 and 380 nm, or a mercury low-pressure gas discharge lamp which emits light with a maximum emission at 254 nm.
Backlighting using a mercury high-pressure lamp having a maximum emission at wavelengths between 360 and 380 nm has the drawback that, apart from short-wave light, also light of substantial intensity is emitted at 408, 435 and 546 nm. This leads to an incomplete division into the three primary colors red, green and blue in the phosphors, and to chromatic aberration of the image displayed on the display screen.
Backlighting using a mercury low-pressure lamp having a maximum emission at a wavelength of 254 nm has the drawback that light of this wavelength is absorbed in the liquid crystal material, leading to photochemical reactions which may destroy the liquid crystal material in the course of time.
Therefore, it is an object of the invention to provide a liquid-crystal color display screen which yields a color-pure image and has a long service life.
In accordance with the invention, this object is achieved by a liquid crystal color display screen, which is provided with an electro-optical medium, two parallel transparent substrates by which the electro-optical medium is flanked, means for influencing the transmission state of the electro-optical medium, a phosphor layer comprising at least two phosphors on the first substrate, and a radiation source for emitting radiation having a maximum emission at a wavelength xcex1 less than 360 nm, which radiation source is situated at the side of the second substrate, and means for transforming the radiation having a maximum emission at a wavelength xcex1 less than 360 nm to radiation having a maximum emission at a wavelength xcex2 greater than 360 nm.
By virtue thereof, the color purity of the color pixels and hence the color-fast, complete color mixture on the color display screen is improved. Photochemical reactions between the backlighting and the electro-optical medium are precluded.
In accordance with a preferred embodiment of the invention, the means for transforming the radiation is a UV phosphor.
The UV phosphor may be selected from the group formed by Ca2,45B5.5P2O15.75:Ce3+Li+; SrB4O7:Eu2+; Sr3(PO4)2:Sn2+;Ba2Mg(BO3)2:Pb2+; LaGd(BO3)2:Ce2+; CaSO4:Eu2+; ZnO:Ga3+; CaO:Bi3+; (Sr,Mg)2P2O7:Eu2+; CaB2P2O9:Eu2+and Sr2P2O7:Eu2+.
In accordance with a particularly preferred embodiment of the invention, the radiation source for radiation having a maximum emission at a wavelength xcex1 less than 360 nm is a mercury low-pressure lamp. A backlighting comprising a mercury low-pressure lamp proved to be very advantageous, as it very efficiently excites the UV phosphors.
It may also be preferred that the electro-optical medium is a liquid crystal cell medium having a 180xc2x0-360xc2x0 twist.
In a variant of the liquid crystal color display screen, the phosphor layer comprises a red phosphor selected from the group Y(V,P,B)O4:Eu; Mg4GeO5.5F:Mn; YNbO4:Eu3+; Y2O2S:Eu3+; Eu(ttfa)3(Clphen)Eu(ttfa)3(phen) and Eu(tfnb)3(dpphen). The luminous intensity in the red range achieved by means of these phosphors, and the resultant optical efficiency, are very high.
In another variant of the liquid crystal color display screen, the phosphor layer comprises a blue phosphor selected from the group BaMgAl10O27:Eu. The luminous intensity in the blue range achieved by means of these phosphors, and the resultant optical efficiency, are very high.
In a further variant of the liquid crystal color display screen, the phosphor layer comprises a green phosphor selected from the group BaMgAl10O27:Mn,Eu; Tb(bph4COO)3(HOEt)2, Tb(dmbtacn)3(HOEt)2 and Tb(benz)3(HOEt)2. The luminous intensity in the green range achieved by means of these phosphors, and the resultant optical efficiency, are very high.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.